1. Field of the Invention
The present invention relates generally to centrifugal compressors, such as a centrifugal supercharger for providing increased airflow to an engine. More particularly, the present invention concerns a compound bearing assembly for a centrifugal compressor. The inventive multiple bearing arrangement is specifically configured for use in extremely high acceleration and velocity conditions and provides relatively low velocity variances among the assembly components.
2. Discussion of Prior Art
The impeller of a centrifugal compressor is often accelerated very quickly and rotated at extremely high speeds. Such conditions are particularly prevalent in some superchargers. For example, in some superchargers, the impeller is accelerated in excess of 80,000 rpm in much less than a second. It is also noteworthy that clearance between the impeller and compressor chamber wall is typically quite small (e.g., less than six thousands of an inch). In this regard, rotation of the impeller must be tightly restricted and controlled about its rotational axis. Impeller xe2x80x9cslopxe2x80x9d is of particular concern in superchargers utilizing a cantilevered impeller (i.e., an impeller fixed on a cantilevered section of a support shaft). The bearing assemblies used to support the impeller shaft must therefore be capable of withstanding high accelerations and velocities without permitting untoward eccentric movement of the impeller relative to the rotational axis.
It is commonly believed that bearing assemblies used in high speed and high acceleration applications must be specially configured and/or formed of unique and expensive materials. For example, traditional high speed bearings often have specially shaped races designed to withstand the tremendous loads typically associated with extreme speed and acceleration applications. Those ordinarily skilled in the art will appreciate that one such bearing assembly (referred to as an xe2x80x9cangular contact bearingxe2x80x9d) includes inner and outer races having raised, diametrically opposed (relative to the balls) projections along opposite sides of the bearing which are engaged by the balls (as opposed to a so-called xe2x80x9cradial bearingxe2x80x9d in which the balls contact the races along only a generally radial line relative to the rotational axis of the bearing). It is also conventional wisdom to form the bearing of high strength, heat resistant materials such as ceramics. Yet further, it is traditionally believed that bearing assemblies used in high speed and acceleration applications should be relatively light. Conventional wisdom further suggests that an axial preload is necessary in high speed conditions to minimize ball skidding and equally load the balls.
Preloading of the bearing is also believed to be particularly important with respect to precisely locating the impeller within the compressor chamber. That is to say, the bearings used to support a high speed impeller shaft of a conventional compressor are highly preloaded (e.g., with a spring) to reduce bearing deflection and take up internal play (e.g., play attributable to manufacturing tolerances and/or inaccuracies). Those skilled in the art will certainly appreciate that impeller xe2x80x9cslop,xe2x80x9d particularly in the case of a cantilevered impeller, is highly undesirable and conventional compressors utilize well-accepted principles to avoid this problem. Moreover, no conventional high speed compressor is known to have utilized a compound bearing to support the impeller shaft, as such a configuration inherently has greater slop than a single bearing and is counterintuitive.
In accordance with the objects evident from the description herein, the present invention concerns a compressor that generally includes a case, a rotatable impeller operable to compress a fluid within the case when rotated, a shaft fixed relative to the impeller, a compound bearing assembly rotatably supporting the shaft on the case. The compound bearing assembly includes a shaft bearing and a case bearing. The shaft and case bearings each include relatively moveable inner and outer races, a cage, and a plurality of balls rotatably positioned within the cage between the races. The inner race of the shaft bearing is fixed relative to the shaft, the outer race of the case bearing is fixed relative to the case, and the outer race of the shaft bearing and inner race of the case are being fixed relative to one another. At least one of the cages of the bearings is selected from the group consisting of a nylon cage that is formed of nylon and presents a plurality ball-receiving openings, and a steel cage that includes a pair of interconnected sections which are formed of steel and cooperatively present a plurality of ball-receiving holes. The at least one of the cages is associated with a respective one of the bearings that has a ball pitch line velocity of at least about 3000 feet per minute (xe2x80x9cfpmxe2x80x9d) during operation of the impeller.
Another aspect of the present invention involves at least one of the shaft and case bearings having a radial configuration, with the races of the at least one of the bearings presenting opposed ball-receiving grooves that are aligned along a radial line that projects at least substantially radially from the rotational axis. In the inventive compressor, the at least one of the bearings has a ball pitch line velocity of at least about 5500 feet per minute during operation of the impeller.
The present invention also concerns forming the balls of both the shaft and case bearings of steel. In the inventive compressor, at least one of the bearings has a ball pitch line velocity of at least about 8000 fpm during operation of the impeller.
Yet another aspect of the present invention involves a unique cage design for a compressor. Particularly, the cage of at least one of the shaft and case bearings includes an annular wall that presents a plurality ball-receiving openings. The inventive cage further includes a pair of radial projections extending at least substantially around the circumference of the wall, with the ball-receiving openings being located between the projections.
The present invention also more specifically concerns a centrifugal supercharger for supercharging an engine of a vehicle. The inventive supercharger generally comprises a case, a rotatable impeller operable to compress induction fluid for the engine when rotated, a shaft fixed relative to the impeller and drivingly connectable to the engine, and a compound bearing assembly rotatably supporting the shaft on the case. The compound bearing assembly includes a radially innermost race fixed relative to the shaft, a radially outermost race fixed relative to the case, and an integrally formed common race assembly spaced radially between the innermost and outermost races. The common race assembly is moveable relative to the innermost and outermost races. The common race assembly presents an inside race associated with the innermost race and an outside race associated with the outermost race. The common race weighs an amount that is equal to or less than about one and one-half times the weight of the innermost race.
In another aspect of the present invention, the inner race of the case bearing and outer race of the shaft bearing are not integrally formed, but rather a coupler is located between and fixedly interconnects the outer race of the shaft bearing and the inner race of the case bearing. The coupler has a coefficient of thermal expansion that is greater than the coefficient(s) of thermal expansion for the outer race of the shaft bearing and the inner race of the case bearing. The coupler also weighs less than each of the outer race of the shaft bearing and the inner race of the case bearing.
The present invention also concerns a method of rotatably mounting an impeller shaft of a centrifugal supercharger in a supercharger case. The method involves providing a case bearing having an inner diameter and a shaft bearing having an outer diameter that is greater than the inner diameter. At least one of the bearings is altered so that the outer diameter of the shaft bearing is smaller than the inner diameter of the case bearing, thereby permitting the shaft bearing to fit within the case bearing. The inner race of the case bearing is fixed to the outer race of the shaft bearing, the outer race of the case bearing is fixed to the case of the supercharger, and the inner race of the shaft bearing if fixed to the impeller shaft.
Another aspect of the present invention involves forming both the shaft and case bearings as ball-type bearings. More particularly, the shaft bearing is a standard 300 Series bearing.
The present invention also involves a supercharger including a pair of spaced apart bearing assemblies supporting the impeller shaft on the case. A first one of the pair of bearing assemblies comprises a compound bearing assembly that includes a first shaft bearing and a radially outward first case bearing. A second one of the pair of bearing assemblies includes a second shaft bearing. The inner race of the first shaft bearing is fixed relative to the shaft, the outer race of the first case bearing is fixed relative to the case, and the outer race of the first shaft bearing and inner race of the first case bearing are fixed relative to one another. In one aspect of the present invention, the inner race of the second shaft bearing is fixed relative to the shaft and the outer race of the second shaft bearing is fixed relative to the case. In another aspect of the present invention, the bearings each include ball ring that includes a cage and a plurality of balls rotatably positioned within the cage between the respective races. Moreover, the second shaft bearing weighs less than the first shaft bearing.
It is another aspect of the present invention to provide a supercharger with a compound bearing assembly for rotatably supporting the impeller shaft, wherein the shaft and case bearings each include relatively moveable inner and outer races, a cage, and a plurality of balls rotatably positioned within the cage between the races. The case bearing presents a case bearing weight. The compound bearing assembly is axially preloaded to a preload amount, wherein the preload amount is determined according to the equation: preload amountxe2x89xa638xc3x97(case bearing weight)0.82.
The present invention alternatively concerns a supercharger having a compound bearing assembly for rotatably supporting the impeller shaft, wherein the bearing assembly is under substantially no axial preload.
A further aspect of the present invention concerns a supercharger having an impeller shaft that presents a cantilevered section, with the impeller being located on the cantilevered section. The inner race of the shaft bearing presents an inside shaft-engaging surface that is rotated at speeds of at least about 5500 fpm during operation of the impeller.
In addition, an aspect of the present invention involves a supercharger having a cantilevered impeller and a compound bearing assembly rotatably supporting the impeller shaft. The bearing assembly includes a shaft bearing and a radially outward case bearing, each of which includes relatively moveable inner and outer races, a cage, and a plurality of balls rotatably positioned within the cage between the races. The balls of the shaft bearing present a common diameter that is equal to or greater than the common diameter of the balls of the case bearing.